Fabrication method of semiconductor integrated circuit device

ABSTRACT

Provided is a fabrication method of a semiconductor integrated circuit device, which comprises disposing, in a ultrapure water preparing system, UF equipment having therein a UF module which has been manufactured by disposing, in a body thereof, a plurality of capillary hollow fiber membranes composed of a polysulfone membrane or polyimide membrane, bonding the plurality of hollow fiber membranes at end portions thereof by hot welding, and by this hot welding, simultaneously adhering the hollow fiber membranes to the body. Upon preparation of ultrapure water to be used for the fabrication of the semiconductor integrated circuit device, the present invention makes it possible to prevent run-off of ionized amine into the ultrapure water.

This application is a Continuation application of application Ser. No.11/062,615, filed Feb. 22, 2005, which is a continuation application ofapplication Ser. No. 10/868,838, filed Jun. 17, 2004, now abandoned,which is a Continuation application of application Ser. No. 10/266,769,filed Oct. 9, 2002, now abandoned, the contents of which areincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of fabrication of asemiconductor integrated circuit device; and, more particularly, theinvention relates to a technique for improving the quality of pure waterto be used for the fabrication of a semiconductor integrated circuitdevice.

Upon fabrication of a semiconductor device, including microfabricationof an integrated circuit, it is required to remove impurities from thesurface or interface of a semiconductor wafer (which will hereinaftersimply be called a “wafer”) by cleaning, thereby maintaining thecleanness of the wafer. Foreign matter on a wafer may causedisconnection or short-circuit of wiring. In particular, heavy metalcomponents must be removed completely, because they have a seriousinfluence on the electrical properties of the device.

Pure water is used for washing a wafer surface to remove a chemicalsolution after the wafer has been cleaned therewith or wet etchedtherewith, thereby making the wafer clean; or pure water is used forpreparing a chemical solution used in a cleaning or wet etching step.Pure water used in such a step is prepared by removing, from raw water,fine particles, organic matter and high molecular ions by use of an RO(reverse osmosis) unit equipped with an RO (Reverse Osmosis) membrane,removing the other ions by using an ion exchange resin, and thenremoving fine particles and living bacteria, which are still in the rawwater after the action of the RO unit and ion exchange resin, using UFequipment (ultrafiltration equipment). A process for preparation of suchpure water is disclosed, for example, in Japanese Unexamined PatentApplication No. Hei 4(1992)-78483. Also, Japanese Unexamined PatentApplication No. Hei 10(1998)-216721 discloses a technique for removinganions, which are too small to pass through UF equipment, using an anionadsorption membrane apparatus disposed downstream of the UF equipment.

SUMMARY OF THE INVENTION

The present inventors have investigated the possibility to construct asystem for obtaining pure water having high purity (which willhereinafter be called “ultrapure water”) to be used for the fabricationof a semiconductor integrated circuit device. During the investigation,they found that the problems as described below occur.

UF equipment is used in the final step of the preparation of ultrapurewater. The UF equipment has a moduled filter obtained by bundling aplurality of capillary hollow fiber membranes using an adhesive whichcontains an epoxy resin as a raw material. This filter needs periodicreplacement with a new one owing to the life of its material. Theadhesive used for bundling the hollow fiber membranes contains an amine,and a portion of this amine has been ionized. When water is caused topass through this UF equipment, after replacement of the filter, thisionized amine is hydrolyzed and is transferred into the ultrapure water.If ultrapure water containing this ionized amine is used, for example,for cleaning a wafer just before the formation of a gate oxide film of aMISFET (Metal Insulator Semiconductor Field Effect Transistor), the Si(silicon) which constitutes the wafer is inevitably etched by thisionized amine, resulting in the formation of an unevenness on theinterface between the gate insulating film and the wafer after formationof the gate insulating film. When the MISFET formed under such a stateconstitutes a memory cell of an electrically erasable programmable readonly memory (EEPROM; which will hereinafter be called “flash memory”),the breakdown voltage of the gate insulating film is lowered, leading tothe problem of a deterioration in the write characteristics to thememory cell, as well as the erase characteristics thereof. Even if theabove-described MISFET is used for semiconductor devices other than thememory cell of a flash memory, an electric current between the sourceand the drain is disturbed, causing a failure in the characteristics.

A test made by the present inventors has revealed that the ionized aminecomes also from the RO unit and ion exchange resin. There is apossibility that such an ionized amine coming from a place other thanthe UF equipment flows into the ultrapure water.

An object of the present invention is to prevent run-off of an ionizedamine into ultrapure water upon preparation of the ultrapure water to beused for the fabrication of a semiconductor integrated circuit device.

The above-described and the other objects and novel features of thepresent invention will be apparent from the following description hereinand the accompanying drawings.

Typical aspects of the invention, among those disclosed in the presentapplication, will be summarized briefly below.

In one aspect of the present invention, there is provided a method offabrication of a semiconductor integrated circuit device, whichcomprises introducing indifferent water, as first raw material water,into a primary pure water system having a primary purifying system;introducing, as second raw material water, the primary pure water, whichhas been obtained by purification through the primary purifying system,into a secondary pure water circulating system having a secondarypurifying system; and feeding a first wet treatment apparatus with thesecondary pure water which has been obtained by purification through thesecondary purifying system, thereby subjecting a semiconductorintegrated circuit wafer to first wetting treatment, wherein, in thesecondary purifying system, there are an ion removing step through useof an ion removing filter, a foreign particle removing step through useof an ultrafiltration filter, and a step of feeding the first wettingtreatment apparatus with pure water which has passed through the ionremoving filter and the ultrafiltration filter, and by the time when thepure water is fed to the first wetting treatment apparatus, ionizedamines or ionized amine substances have been removed from the secondarypure water to such an extent as not to affect the characteristics of thesemiconductor integrated circuit device.

In another aspect of the present invention, there is also provided amethod of fabrication of a semiconductor integrated circuit device,which comprises introducing, as first raw material water, indifferentwater into a primary pure water system having a primary purifyingsystem; introducing, as second raw material water, the primary water,which has been obtained through the primary purifying system, into asecondary pure water circulating system having a secondary purifyingsystem; and feeding a first wetting treatment apparatus with thesecondary pure water which has been obtained by purification of thesecond raw material water through the secondary purifying system,thereby subjecting a semiconductor integrated circuit wafer to firstwetting treatment; wherein, in the secondary purifying system, there area step of removing foreign particles from pure water through use of anultrafiltration filter; a step of removing ions from the pure water,which has passed through the ultrafiltration filter, by use of amembrane type ion removing filter; and a step of feeding the firstwetting treatment apparatus with the pure water which has passed throughthe ion removing filter.

In a further aspect of the present invention, there is also provided amethod of fabrication of a semiconductor integrated circuit device,which comprises introducing, as first raw material water, indifferentwater into a primary pure water system having a primary purifyingsystem; introducing, as second raw material water, the primary water,which has been obtained through the primary purifying system, into asecondary pure water circulating system having a secondary purifyingsystem; and feeding a first wetting treatment apparatus with secondarypure water which has been obtained by purification through the secondarypurifying system, thereby subjecting a semiconductor integrated circuitwafer to first wetting treatment; wherein, in the secondary purifyingsystem, there are a step of removing foreign particles from pure waterthrough use of an ultrafiltration filter disposed in the secondarypurifying system; a step of removing ions from the pure water, which haspassed through the ultrafiltration filter, by use of a membrane type ionremoving filter that is disposed outside of the secondary pure watercirculating system; and a step of feeding the first wetting treatmentapparatus with the pure water which has passed through the ion removingfilter.

In a still further aspect of the present invention, there is alsoprovided a method of fabrication of a semiconductor integrated circuitdevice, which comprises introducing, as first raw material water,indifferent water into a primary pure water system having a primarypurifying system; introducing, as second raw material water, the primarywater, which has been obtained through the primary purifying system,into a secondary pure water circulating system having a secondarypurifying system; and feeding a first wetting treatment apparatus withthe secondary pure water which has been obtained by purification throughthe secondary purifying system, thereby subjecting a semiconductorintegrated circuit wafer to first wetting treatment, wherein: in thesecondary purifying system, there are a step of removing ions from purewater through use of an ion removing filter that is disposed inside ofthe secondary purifying system; a step of causing the pure water, whichhas passed through the ion removing filter, to pass through a heatwelding type ultrafiltration filter that is disposed inside of thesecondary purifying system, thereby removing foreign particles from thepure water; and a step of feeding the first wetting treatment apparatuswith the pure water which has passed through the ultrafiltration filter.

In a still further aspect of the present invention, there is alsoprovided a method of fabrication of a semiconductor integrated circuitdevice, which comprises introducing, as first raw material water,indifferent water into a primary pure water system having a primarypurifying system; introducing, as second raw material water, the primarywater, which has been obtained through the primary purifying system,into a secondary pure water circulating system having a secondarypurifying system; and feeding a first wetting treatment apparatus withthe secondary pure water which has been obtained by purification throughthe secondary purifying system, thereby subjecting a semiconductorintegrated circuit wafer to first wetting treatment; wherein, in thesecondary purifying system, there are a step of removing ions by use ofan ion removing filter, a step of removing foreign particles through aultrafiltration filter, and a step of feeding the first wettingtreatment apparatus with the pure water which has passed through the ionremoving filter and the ultrafiltration filter, and wherein theultrafiltration filter is disposed at a position permitting selfcleaning.

The outline of other examples of the invention described in the presentapplication will be described below:

Item 1: A method of fabrication of a semiconductor integrated circuitdevice, which comprises cleaning a semiconductor substrate or preparinga chemical solution with pure water prepared by a pure water preparationstep having the sub-steps of:

(a) removing first foreign matter from raw water containing foreignmatter, and

(b) removing, after the sub-step (a), foreign matter other than thefirst foreign matter from the raw water by using a first apparatusequipped with a filter formed by bonding a plurality of hollow fibermembranes at the end portions thereof, wherein the hollow fibermembranes permit passage of only substances having a molecular weightnot greater than a predetermined value, the plurality of hollow fibermembranes are heat welded or bonded with an amine-free material, and thefirst apparatus removes the foreign matter other than the first foreignmatter from the raw water by causing the raw water to pass through thefilter.

Item 2: The method of fabrication of a semiconductor integrated circuitdevice according to Item 1, wherein the hollow fiber state membranes areeach composed mainly of polysulfone or polyimide.

Item 3: The method of fabrication of a semiconductor integrated circuitdevice according to Item 1, further comprising heat treating thesemiconductor substrate after the cleaning step, thereby forming a gateinsulating film.

Item 4: The method of fabrication of a semiconductor integrated circuitdevice according to Item 3, wherein the gate insulating film is formedto have a film thickness of 20 nm or less.

Item 5: The method of fabrication of a semiconductor integrated circuitdevice according to Item 1, further comprising, after the cleaning step,forming a nonvolatile memory cell, the nonvolatile memory cell formingstep having the following sub-steps:

(c) heat treating the semiconductor substrate, thereby forming a gateinsulating film,

(d) forming thereover a first conductive film,

(e) forming thereover a first insulating film,

(f) forming thereover a second conductive film,

(g) patterning the second conductive film, thereby forming a controlgate electrode made thereof, and

(h) patterning the first insulating film and the first conductive film,thereby forming a floating gate electrode made of the first conductivefilm.

Item 6: The method of fabrication of a semiconductor integrated circuitdevice according to Item 5, wherein the gate insulating film is formedto have a thickness of 10 nm or less.

Item 7: The method of fabrication of a semiconductor integrated circuitdevice, which comprises:

(a) removing first foreign matter from raw water containing foreignmatter,

(b) after the step (a), removing foreign matter other than the firstforeign matter from the raw water by using a first apparatus equippedwith a filter formed by bonding a plurality of hollow fiber membranes atend portions thereof, and

(c) after the step (b), cleaning a semiconductor substrate or preparinga chemical solution with pure water prepared by causing the raw water topass through a first filter made of a hollow-fiber-type filter membranehaving an ion exchange radical, thereby removing ionized amines from theraw water, wherein, the first apparatus is capable of removing foreignmatter other than the first foreign matter from the raw water by causingthe raw water to pass through the filter.

Item 8: The method of fabrication of a semiconductor integrated circuitdevice according to Item 7, wherein the step (a) includes a sub-step ofremoving ions from the raw water through a second filter made of an ionexchange resin having an ion exchange radical or a hollow-fiber-typefilter membrane having an ion exchange radical.

Item 9: The method of fabrication of a semiconductor integrated circuitdevice according to Item 7, which further comprises forming a gateinsulating film by heat treating the semiconductor substrate aftercleaning.

Item 10: The method of fabrication of a semiconductor integrated circuitdevice according to Item 9, wherein the gate insulating film is formedto have a film thickness of 20 nm or less.

Item 11: The method of fabrication of a semiconductor integrated circuitdevice according to Item 7, which further comprises forming anonvolatile memory cell after the cleaning step, the nonvolatile memorycell forming step having the following sub-steps:

(c) heat treating the semiconductor substrate, thereby forming a gateinsulating film,

(d) forming thereover a first conductive film,

(e) forming thereover a first insulating film,

(f) forming thereover a second conductive film,

(g) patterning the second conductive film to form a control gateelectrode made of the second conductive film, and

(h) patterning the first insulating film and first conductive film toform a floating gate electrode made of the first conductive film.

Item 12: The method of fabrication of a semiconductor integrated circuitdevice according to Item 11, wherein the gate insulating film is formedto have a thickness of 10 nm or less.

Item 13: A method of fabrication of a semiconductor integrated circuitdevice, which comprises cleaning a semiconductor substrate or preparinga chemical solution with pure water prepared through a pure waterpreparing step, comprising the sub-steps of:

(a) removing first foreign matter from raw water containing foreignmatter, and

(b) after the step (a), removing foreign matter other than the firstforeign matter by use of a first apparatus equipped with a filter formedby bonding a plurality of hollow fiber membranes at end portionsthereof, wherein the sub-step (a) further comprises removing ions fromthe raw water through use of a second filter made of a hollow-fiber-typefilter membrane having an ion exchange radical.

Item 14: The method of fabrication of a semiconductor integrated circuitdevice according to Item 13, wherein the semiconductor substrate is heattreated after the cleaning step, thereby forming a gate insulating film.

Item 15: The method of fabrication of a semiconductor integrated circuitdevice according to Item 14, wherein the gate insulating film is formedto have a thickness of 20 nm or less.

Item 16: The method of fabrication of a semiconductor integrated circuitdevice according to Item 13, which further comprises forming anonvolatile memory cell after the cleaning step, thenon-volatile-memory-cell forming step including the following sub-steps:

(c) heat treating the semiconductor substrate, thereby forming a gateinsulating film,

(d) forming thereover a first conductive film,

(e) forming thereover a first insulating film,

(f) forming thereover a second conductive film,

(g) patterning the second conductive film to form a control gateelectrode made of the second conductive film, and

(h) patterning the first insulating film and first conductive film toform a floating gate electrode made of the first conductive film.

Item 17: The method of fabrication of a semiconductor integrated circuitdevice according to Item 16, wherein the gate insulating film is formedto have a film thickness of 10 nm or less.

Item 18: A method of fabrication of a semiconductor integrated circuitdevice, which comprises cleaning a semiconductor substrate or preparinga chemical solution with pure water prepared through a pure waterpreparing step comprising the sub-steps of:

(a) removing first foreign matter from raw water containing foreignmatter, and

(b) after the step (a), removing foreign matter other than the firstforeign matter by use of a first apparatus equipped with a filter formedby bonding a plurality of hollow fiber membranes at end portionsthereof; wherein, in a pathway for sending the pure water from the firstapparatus to an apparatus in which the cleaning step or chemicalsolution preparing step is conducted, a first filter made of ahollow-yarn type filter membrane having an ion exchange radical or anion exchange resin having an ion exchange radical is disposed; and

wherein ionized amines are removed from the pure water through use ofthe first filter.

Item 19: The method of fabrication of a semiconductor integrated circuitdevice according to Item 18, further comprising heat treating thesemiconductor substrate after the cleaning step, thereby forming a gateinsulating film.

Item 20: The method of fabrication of a semiconductor integrated circuitdevice according to Item 19, wherein the gate insulating film is formedto have a film thickness of 20 nm or less.

Item 21: The method of fabrication of a semiconductor integrated circuitdevice according to Item 18, which further comprises forming anonvolatile memory cell after the cleaning step, thenon-volatile-memory-cell forming step including:

(c) heat treating the semiconductor substrate, thereby forming a gateinsulating film,

(d) forming thereover a first conductive film,

(e) forming thereover a first insulating film,

(f) forming thereover a second conductive film,

(g) patterning the second conductive film to form a control gateelectrode made of the second conductive film, and

(h) patterning the first insulating film and first conductive film toform a floating gate electrode made of the first conductive film.

Item 22: The method of fabrication of a semiconductor integrated circuitdevice according to Item 21, wherein the gate insulating film is formedto have a film thickness of 10 nm or less.

Item 23: A method of fabrication of a semiconductor integrated circuitdevice, which comprises cleaning a semiconductor substrate or preparinga chemical solution with pure water prepared through use of a pure waterpreparing step, comprising sub-steps of:

(a) removing first foreign matter from raw water containing foreignmatter, and

(b) after the step (a), removing foreign matter other than the firstforeign matter by use of a plurality of first apparatuses each equippedwith a filter formed by bonding a plurality of hollow fiber membranes atend portions thereof, wherein:

the step (a) further comprises:

(a1) removing ions from the raw water by use of a second filter made ofan ion exchange resin having an ion exchange radical or ahollow-fiber-type filtration membrane having an ion exchange radical;

the step (a) is followed by at least one of the sub-steps of:

(c) causing a portion of the raw water, which has passed through thesecond filter, to pass through a new one of the first apparatus or a newone of the second filter and then feeding the resulting purified waterto the second filter, and

(d) causing the residue of the pure water, after the cleaning step orchemical-solution-preparing step, to pass through at least one of a newone of the first apparatus or a new one of the second filter, and thenfeeding to the second filter; and

the steps (c) and/or (d) are carried out for a predetermined term.

Item 24. The method of fabrication of a semiconductor integrated circuitdevice according to Item 23, further comprising forming a gateinsulating film by heat treating the semiconductor substrate after thecleaning step.

Item 25. The method of fabrication of a semiconductor integrated circuitdevice according to Item 24, wherein the gate insulating film is formedto have a film thickness of 20 nm or less.

Item 26. The method of fabrication of a semiconductor integrated circuitdevice according to Item 23, which further comprises forming anonvolatile memory cell after the cleaning step, thenon-volatile-memory-cell forming step including:

(c) heat treating the semiconductor substrate, thereby forming a gateinsulating film,

(d) forming thereover a first conductive film,

(e) forming thereover a first insulating film,

(f) forming thereover a second conductive film,

(g) patterning the second conductive film to form a control gateelectrode made of the second conductive film, and

(h) patterning the first insulating film and the first conductive filmto form a floating gate electrode made of the first conductive film.

Item 27: The method of fabrication of a semiconductor integrated circuitdevice according to Item 26, wherein the gate insulating film is formedto have a film thickness of 10 nm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross-sectional view illustrating a step in thefabrication of a semiconductor integrated circuit device according toone embodiment of the present invention;

FIG. 2 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 2;

FIG. 4 is a schematic diagram illustrating the system used in preparingultrapure water to be used for the fabrication of the semiconductorintegrated circuit device according to the one embodiment of the presentinvention;

FIG. 5 is a schematic diagram illustrating details of the ultrapurewater preparing system illustrated in FIG. 4;

FIG. 6 is a schematic cross-sectional view of a UF module of UFequipment included in the system used on the preparation of ultrapurewater to be used for the fabrication of the semiconductor integratedcircuit device according to the one embodiment of the present invention;

FIG. 7 is a fragmentary cross-sectional view of the UF module, takenalong line A-A in FIG. 6;

FIG. 8 is a schematic diagram of a hollow fiber membrane constitutingthe UF module illustrated in FIG. 6;

FIG. 9 is a schematic diagram of an ion filter to be disposed downstreamof the UF equipment included in the system for preparing ultrapure waterto be used for the fabrication of the semiconductor integrated circuitdevice according to the one embodiment of the present invention;

FIG. 10 is a fragmentary cross-sectional view illustrating how ions aretrapped by the ion filter illustrated in FIG. 9;

FIG. 11 is a diagram which illustrates one example of the ion filterillustrated in FIG. 9;

FIG. 12 is a diagram which illustrates one example of the ion filterillustrated in FIG. 9;

FIG. 13 is a diagram which illustrates another example of the ion filterillustrated in FIG. 9;

FIG. 14 is a diagram which illustrates a further example of the ionfilter illustrated in FIG. 9;

FIG. 15 is a schematic diagram illustrating the constitution of the UFequipment included in the system for preparing ultrapure water to beused for the fabrication of the semiconductor integrated circuit deviceaccording to the one embodiment of the present invention;

FIG. 16 is a schematic diagram for illustrating an anion deminer and acation deminer included in the system for preparing ultrapure water tobe used for the fabrication of the semiconductor integrated circuitdevice according to the one embodiment of the present invention;

FIG. 17 is a schematic diagram illustrating ion adsorption by an ionexchange resin as shown in FIG. 16;

FIG. 18 is a schematic diagram of one example of a cleaning and draftingapparatus to be used for the fabrication of the semiconductor integratedcircuit device according to the one embodiment of the present invention;

FIG. 19 is a schematic diagram of a dilute hydrofluoric acid preparingapparatus for preparing dilute hydrofluoric acid to be fed to thecleaning and drafting apparatus shown in FIG. 18;

FIG. 20 is a schematic diagram of one example of a wet etching apparatusto be used for the fabrication of the semiconductor integrated circuitdevice according to the one embodiment of the present invention;

FIG. 21 is a schematic diagram of one example of a cleaning and draftingapparatus to be used for the fabrication of the semiconductor integratedcircuit device according to the one embodiment of the present invention;

FIG. 22 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 3;

FIG. 23 is a fragmentary cross-sectional view illustrating the shape ofthe interface between the semiconductor substrate and the gateinsulating film formed thereover after cleaning with ultrapure waterhaving ionized amines mixed therein;

FIG. 24 is a fragmentary cross-sectional view illustrating the shape ofthe interface between the semiconductor substrate and the gateinsulating film formed thereover after cleaning with ultrapure waterfree of ionized amines;

FIG. 25 is a schematic diagram illustrating a method of measuring thebreakdown voltage of the gate insulating film of MISFET of thesemiconductor integrated circuit device according to the one embodimentof the present invention;

FIG. 26 is a schematic diagram illustrating the results of measurementof the breakdown voltage of the gate insulating film when thesemiconductor substrate is cleaned with ultrapure water prepared justafter replacement of the UF of the UF equipment with a new one;

FIG. 27 is a schematic diagram illustrating the results of measurementof the breakdown voltage of the gate insulating film when thesemiconductor substrate is cleaned with ultrapure water prepared justafter replacement, with new ones, of an ion exchange resin type anionremoving filter and an ion exchange resin type cation removing filter,each included in the system for preparation of ultrapure water to beused for the fabrication of the semiconductor integrated circuit deviceaccording to the one embodiment of the present invention;

FIG. 28 is a schematic diagram illustrating the results of measurementof the breakdown voltage of the gate insulating film when thesemiconductor substrate is cleaned with ultrapure water prepared using aUF of the UF equipment which has been used for a long time;

FIG. 29 is a schematic diagram illustrating the results of measurementof the breakdown voltage of the gate insulating film when thesemiconductor substrate is cleaned with ultrapure water prepared usingUF equipment having a UF replaced with a new one and having a mixdeminer disposed downstream of the equipment;

FIG. 30 is a schematic diagram illustrating the results of measurementof the breakdown voltage of the gate insulating film when thesemiconductor substrate is cleaned with ultrapure water prepared usingUF equipment having a UF replaced with a new one and having, downstreamof the equipment, an ion filter with a membrane film;

FIG. 31 is a schematic diagram illustrating the relationship between theamount of ionized amines attached to the semiconductor substrate bycleaning with ultrapure water and the existence or absence of adefective gate insulating film;

FIG. 32 is a schematic diagram illustrating the relationship between thecleaning date of the semiconductor substrate with ultrapure water andpercent of defective gate insulating films;

FIG. 33 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 22;

FIG. 34 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 33;

FIG. 35 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 34;

FIG. 36 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 35;

FIG. 37 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 36;

FIG. 38 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 39;

FIG. 39 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 38; and

FIG. 40 is a fragmentary cross-sectional view of the semiconductorintegrated circuit device during the fabrication step following the stepof FIG. 39.

DETAILED DESCRIPTION OF THE INVENTION

Prior to a detailed description of the present invention, the meaningsof some terms used in this application will be explained below.

The term “wafer” means a single crystalline Si substrate (generally,having a nearly flat and circular shape), a sapphire substrate, a glasssubstrate or any other insulating, semi-insulating or semiconductorsubstrate and a composite substrate thereof of the type used forfabricating semiconductor integrated circuit devices. Further, the term“semiconductor integrated circuit device” as used herein means not onlythose devices fabricated on a semiconductor or insulating substrate,such as a silicon wafer or sapphire substrate, but also those devicesfabricated on another insulating substrate, such as glass, for example,TFT (Thin-Film-Transistor) or STN (Super-Twisted-Nematic) liquidcrystals, unless otherwise specifically indicated.

The term “device surface” means a main surface of a substrate on whichdevice patterns corresponding to a plurality of chip regions are formedby lithography.

The term “resist pattern” means a film pattern obtained by patterning aphotosensitive resin film (resist film) by photolithography. Thispattern includes a simple resist film free of openings at such aportion.

The term “UF equipment” (Ultrafiltration Equipment) means pressurefiltration equipment for separating molecules according to their sizethrough use of an ultra filter (UF). In this equipment, separation iscarried out at a molecular cutoff range of about thousands to hundredsof thousands. The term “ultra filter” embraces a hollow fiber type ultrafilter and a spiral type ultra filter.

The term “ion exchange resin” means a synthetic resin having a capacityof adsorbing thereto ions existing in water, thereby removing them fromthe water. It can be classified into two types, that is, a cationexchange resin for adsorbing and removing cations (Na⁺, Ca²⁺, Mg²⁺,etc.) and an anion exchange resin for adsorbing and removing anions(Cl⁻, SO₄ ²⁻, SiO₂, etc.). The term “ion exchange resin type ionremoving filter” embraces a cation removing filter for removing cations,an anion removing filter for removing anions and a mixed ion removingfilter for removing both cations and anions.

The term “RO unit” (Reverse Osmosis unit) means an apparatus forremoving ions, organic matter, fine particles and living bacteria inwater through use of an RO film, which is a filter membrane to whichreverse osmosis has been applied.

The term “vacuum degasifier” means an apparatus for spraying water in avacuum atmosphere, thereby removing a dissolved gas in water.

The term “indifferent water” means water which will serve as a rawmaterial for obtaining high purity water to be used for the fabricationof a semiconductor integrated circuit device. River water, ground water(including well water) or the like is employed for this purpose.

The term “primary pure water” means high purity water from which almostall of the impurities, such as ions, fine particles, microorganisms andorganic matter, have been removed from treatment water (indifferentwater).

The term “ultrapure water” means water which is obtained by removing atrace of impurities, such as fine particles, living bacteria, TOC (totalorganic carbon), ions and dissolved oxygen remaining in primary water,thereby having a markedly high purity, and which is to be used, forexample, for the cleaning of a wafer.

The term “primary pure water unit” means one of a unit constituting anultrapure water preparing system. It is formed of a reverse osmosisunit, an ion exchanging apparatus and a degasifier, and it preparesprimary pure water by removing almost all of the impurities, such asfine particles, ions, microorganisms and organic matters, from waterpassing through a pretreatment apparatus.

The term “pretreatment system” means a system including apparatuses forremoving colloidal matter, particulate matter and bacteria from rawwater by physical and chemical treatments prior to the feeding of theraw water to a primary pure water unit.

The term “subsystem” means a system which is disposed in the vicinity ofa point of use and prepares ultrapure water by using, as raw water,primary pure water. It comprises a UV sterilizer, a cartridge polisherand a pressure filter.

The term “ultrapure water preparing system” means a system for preparinghigh purity water by separating impurities from raw water, such as tapwater, industrial water, well water or river water, through use of anion exchange resin membrane or a filter membrane, thereby purifying it.The system comprises a pretreatment unit, a primary pure water unit anda subsystem.

The term “point of use” means a site at which ultrapure water fed from asubsystem is taken out for the purpose of wafer cleaning and providedfor use.

The term “TOC (total organic carbon)” means an organic carbon containedin ultrapure water, and it embraces that material produced from rawwater (natural water or recovered water) or that has escaped from themembers being used, such as an ion exchange resin or pipe.

The term “ultrafiltration membrane” means a plastic porous thin-filmfilter having numerous uniform pores, and which is made of cellulosenitrate, cellulose acetate, acetyl cellulose, nitrocellulose, nylon,Teflon, polyvinyl chloride or ethylene tetrafluoride resin.

The term “new” means an apparatus or member which has not been used, andit also embraces one that has been used for a predetermined term. Withregards to the UF referred to in the below-described embodiment, thispredetermined term corresponds to the term until discharge of ionizedamine outside the UF equipment terminates in the case where the UF ismade of an amine-containing material. This predetermined term varies,depending on the specification of the UF or the amount of water fed tothe UF. In the below-described embodiment, the predetermined term isabout 1 month, preferably about 2 months, more preferably about 3months, after its use is started.

In the below-described embodiment, when reference is made to a number ofelements (including the number, value, amount and range), the number ofelements is not limited to a specific number, but can be greater than orless than the specific number, unless otherwise specifically indicated,or in the case where it is principally apparent that the number islimited to the specific number.

Moreover in the below-described embodiment, it is needless to say thatthe constituting elements (including element steps) are not alwaysessential unless otherwise specifically indicated or in the case whereit is principally apparent that they are essential.

Similarly, in the below-described embodiment, when a reference is madeto the shape or positional relationship of the constituting elements,that which is substantially analogous or similar to it is also embraced,unless otherwise specifically indicated, or in the case where it isutterly different in principle. This also applies to the above-describedvalue and range.

In all the drawings for describing the below-described embodiment,members having a like function will be identified by like referencenumerals and overlapping descriptions thereof will be omitted

In the below-described embodiments, MISFET (Metal InsulatorSemiconductor Field Effect Transistor) representing an electric fieldtransistor will be abbreviated as MIS, while a p-channel type MISFET andan n-channel type MISFET will be abbreviated as pMIS and nMIS,respectively.

The embodiments of the present invention will hereinafter be describedspecifically based on the accompanying drawings.

In this embodiment, the present invention is applied to a method offabrication of a flash memory (semiconductor integrated circuit device).This method of fabrication of a flash memory will be described next inthe order of the steps thereof with reference to FIGS. 1 to 41.

As illustrated in FIG. 1, a semiconductor substrate (semiconductorintegrated circuit wafer) 1 on which the flash memory of this embodimentis to be formed has, for example, a region 1A in which a 5V type nMIS isformed, a region 1B in which a 5V type pMIS is formed, a region 1C inwhich a MIS which is to be a memory cell of a flash memory is formed, aregion 1D in which a high breakdown-voltage one-side offset nMIS isformed, a region 1E2 in which a high breakdown-voltage loading nMIS isformed and a region 1F in which a high breakdown-voltage one-side offsetpMIS is formed.

First, the semiconductor substrate 1 made of p type single crystalsilicon Si is cleaned with dilute hydrofluoric acid (HF) and ultrapurewater, followed by oxidizing treatment on the surface of the substrateto form a silicon oxide film 2A thereover. After deposition of a siliconnitride film (not illustrated) over the silicon oxide film 2A, thesilicon nitride film is etched to selectively leave the silicon nitridefilm over the silicon oxide film 2A.

Using the resulting silicon nitride film as a mask, an impurity (forexample, P (phosphorus)) having an n type conductivity is introducedinto the semiconductor substrate 1 by ion implantation. Afterselectively thickening, by oxidizing treatment, a portion of the siliconoxide film 2A in a region having no silicon nitride film formedthereover, the silicon nitride film is removed using, for example, hotphosphoric acid. The semiconductor substrate 1 is then cleaned withNH₄OH (ammonium hydroxide)/H₂O₂ (hydrogen peroxide)/H₂O, dilutehydrofluoric acid and ultrapure water. The substrate 1 is heat treatedto diffuse the above-described impurity, whereby an n type isolationregion NiSO is formed.

As illustrated in FIG. 2, after cleaning the semiconductor substrate 1with dilute hydrofluoric acid and ultrapure water, oxidizing treatmentis conducted on the surface of the substrate to form a silicon oxidefilm 2 thereover. Using a photoresist film (not illustrated) patternedby photolithography as a mask, an impurity (for example, P) having an ntype conductivity is introduced into the semiconductor substrate 1 byion implantation. After removal of the photoresist film, using anotherphotoresist film (not illustrated) patterned by photolithography as amask, an impurity (for example, BF₂ (boron difluoride) having a p typeconductivity is introduced into the semiconductor substrate 1 by ionimplantation. The semiconductor substrate 1 is then cleaned withNH₄OH/H₂O₂/H₂O, hydrofluoric acid and ultrapure water, followed by heattreatment to diffuse these impurities, whereby an n type well 3 and a ptype well 4 are formed.

As illustrated in FIG. 3, the surface of the semiconductor substrate 1is oxidized to form a silicon oxide film (not illustrated) thereover.After deposition of a silicon nitride film (not illustrated) over thesilicon oxide film, the silicon nitride film is etched with aphotoresist film (not illustrated) as a mask to selectively leave thesilicon nitride film over the silicon oxide film. The photoresist filmis removed. The semiconductor substrate 1 is then cleaned withNH₄OH/H₂O₂/H₂O, followed by further cleaning with HCl/H₂O₂/H₂O. By aselective oxidization method, a field insulating film 6 for elementisolation is formed over the surface of the semiconductor substrate 1.

Using a photoresist film patterned by photolithography as a mask, animpurity (for example, BF₂) having a p type conductivity is introducedby ion implantation. The impurity is diffused by heat treatment, wherebya p type channel stopper region 7 is formed. The silicon nitride filmremaining on the semiconductor substrate 1 is then removed using, forexample, hot phosphoric acid.

The semiconductor substrate 1 is then cleaned with dilute hydrofluoricacid and ultrapure water. The ultrapure water used in this Embodiment isprepared by a system as illustrated in FIGS. 4 and 5. FIG. 4 is aschematic diagram illustrating the outline of the ultrapure waterpreparing system of this Embodiment, while FIG. 5 is a schematic diagramillustrating one example of the details of the ultrapure water preparingsystem illustrated in FIG. 4. A technique relating to such a ultrapurewater preparing system is also described in Japanese Patent ApplicationNo. 2001-314813 by the present inventors.

As illustrated in FIGS. 4 and 5, by the pretreatment system (primarypurifying system) PTS, groundwater (indifferent water (first rawmaterial water) which will hereinafter be called “raw water”) pumped upfrom a well is subjected to chemical and physical treatments to removecolloidal matter (first foreign matter), particulate matter (firstforeign matter) and bacteria (first foreign matter) from the raw water.By use of an RO unit (primary purifying system) RO1, fine particles(first foreign matter), organic matter (first foreign matter), bacteria(first foreign matter) and high molecular ions (first foreign matter)are removed from the raw water. By use of an ion exchange resin typecation removing filter (primary purifying system) CED1, cations (firstforeign matter) are removed from the raw water, followed by removal of adissolved gas in the raw water by a vacuum degasifier (VD). By an ionexchange resin type anion removing filter (primary purifying system)AED1, anions (first foreign matter) are removed from the raw water.After removal of cations (first foreign matter) from the raw water by anion exchange resin type cation removing filter (primary purifyingsystem), anions are removed from the raw water by an ion exchange resintype anion removing filter (primary purifying system) AED2. Downstreamthereof, an RO unit RO2 (not illustrated in FIG. 5) may be disposed toremove, from the raw water, fine particles generated from the anion andcation removing filters. Through the above-described steps, primary purewater can be prepared from the raw water. The primary pure water system(pretreatment system) described here is made up of units used forpreparing primary pure water from the raw water.

The primary pure water (second raw material water) thus prepared is thenfed to an intermediate storage tank (secondary purifying system) MIDT,followed by delivery to a heat exchanger (secondary purifying system)HEXC by a pump (secondary purifying system) PUMP. While the primary purewater is kept at a fixed temperature by means of the heat exchangerHEXC, it is fed to an UV sterilizer (secondary purifying system) UVO1 ora low-pressure UV oxidizer (secondary purifying system) UVO2, in whichthe primary pure water is oxidized or sterilized by exposure to UV rays.The primary pure water sterilized by the UV sterilizer UVO1 is caused topass through an ion exchange resin type mixed ion removing filter(secondary purifying system) MED to remove cations and anions, and itthen delivered to UF equipment (first equipment) UFE. Fine particles andthe like, which cannot be removed by the RO unit and ion removingfilter, can be removed by the UF equipment UFE, making it possible toprepare ultrapure water (secondary pure water) to be used for thefabrication of the semiconductor integrated circuit device of thisEmbodiment and to feed the thus prepared ultrapure water to a point ofuse USEP. The secondary pure water system (subsystem (secondary purewater circulating system)) is formed of each equipment for preparingultrapure water from primary pure water and point of use USEP.

Of the ultrapure water sent to the point of use USEP, a portion of itwhich has not been used up at the point of use USEP can be returned tothe intermediate storage tank MIDT for recycling. Of the ultrapure waterused at the point of use USEP (which water will hereinafter be called“wastewater”), that which is re-usable as ultrapure water is subjectedto ion exchange to remove cations and anions. The wastewater is thensubjected to sterilizing treatment and treatment for removal ofimpurities, such as fine particles, organic matter, bacteria and highmolecular ions, by using an RO unit RO3 having a sterilizing capacity byexposure to ultraviolet rays and a fine-particle removing capacitythrough use of an RO membrane. After various treatments as describedabove, the wastewater, together with the raw material treated by the ROunit RO1, is sent to the cation removing filter CED1. After these steps,a portion of the wastewater becomes re-usable as ultrapure water.

FIG. 6 is a schematic diagram of a UF module of the UF equipment UFEillustrated in FIGS. 4 and 5. FIG. 7 is a cross-sectional view takenalong a line A-A of FIG. 6. The UF module in this Embodiment is made bydisposing, in a body KOT, a plurality of capillary hollow fibermembranes TYM that are formed from a polysulfone membrane or polyimidemembrane, bonding these plurality of hollow fiber membranes TYM at endportions thereof by hot welding, and by this hot welding, adhering thesehollow fiber membranes TYM to the body. As illustrated in FIG. 8, thehollow fiber membranes are each made of a polysulfone membrane orpolyimide membrane so that water, ion molecules and low molecules canpenetrate inside of the hollow fiber membranes TYM, but high moleculescannot. Since the plurality of hollow fiber membranes TYM are hot weldedto each other at end portions thereof in the body KOT and the hollowfiber membranes TYM are adhered to the body, there is discharged fromthe UF module only primary pure water which has penetrated inside of thehollow fiber membranes TYM, and thereby it is deprived of highmolecules, that is, ultrapure water.

When the plurality of hollow fiber membranes TYM are bonded to eachother at end portions thereof by an adhesive containing an epoxy resinas its raw material, the adhesive typically contains an amine, and aportion of this amine exists in an ionized form. In this Embodiment, onthe other hand, the plurality of hollow fiber membranes TYM are hotwelded at end portions thereof so that the adhered portions do notcontain an amine. Therefore, use of the UF module of this Embodimentmakes it possible to prevent the discharge of an ionized amine whichwill otherwise occur when primary pure water is fed to the UF module andthe ionized amine is hydrophilized and then, discharged as a mixturewith ultrapure water. Even if ultrapure water prepared by the ultrapurewater preparing system according to this Embodiment is used for acleaning step of the semiconductor substrate 1 just before the formationof a gate oxide film of the MISFET, which will be a memory cell of aflash memory, it is possible to prevent an inconvenience, such as theformation of an unevenness on the interface between a gate oxide filmand the semiconductor substrate 1 after formation of the gate oxidefilm, which will otherwise be caused by etching of the semiconductorsubstrate by the ionized amine. This results in the prevention oflowering in the breakdown voltage of the gate oxide film, thereby makingit possible to prevent deterioration in the write characteristics anderase characteristics. Lowering in the breakdown voltage of the gateoxide film can be prevented so that even in a MISFET other than a memorycell, the smooth flow of electric current between the source and drainis not disturbed. In this Embodiment, the plurality of hollow fibermembranes TYM are bonded to each other by hot welding, but hot weldingmay be replaced with bonding via an amine-free urethane material.

Downstream of the UF equipment UFE (refer to FIGS. 4 and 5), an ionfilter (first filter), having a membrane film MBF in a circular sheetform, may be disposed as illustrated in FIG. 9. The ultrapure waterpassing through the UF equipment UFE is supplied to its ion filter andthen, enters into the membrane film MBF from a membrane hole MBH of themembrane film MBF. As illustrated in FIG. 10, an ion exchange radicalIER has been formed in the membrane hole MBH. Ions in the ultrapurewater are adsorbed to this ion exchange radical IER and thus can beremoved. In other words, even if a plurality of hollow fiber membranesTYM disposed in the UF module of the UF equipment UFE (refer to FIGS. 4and 5) are bonded to each other at end portions thereof by anamine-containing adhesive (for example, an epoxy resin) and the ionizedamine is discharged together with ultrapure water, it can be removedfrom the ultrapure water by causing the water to pass through theabove-described ion filter.

As illustrated in FIG. 11, it is possible to omit the anion removingfilter AED3 and mixed ion removing filter MED from the ultrapurepreparing system of this Embodiment, as illustrated in FIGS. 4 and 5,and to dispose an ion filter, as illustrated in FIG. 9, downstream ofthe UF equipment. In FIG. 11, the heat exchanger HEXS is notillustrated. When the system does not include these filters, an ionfilter IFA having an ion exchange radical capable of adsorbing theretoanions and an ion filter IFC having an ion exchange radical capable ofadsorbing thereto cations are disposed as the ion filter. Without usingthe anion removing filter AED3 and mixed ion removing filter MED, anionsand cations can be removed from the primary pure water by the ion filterIFA and ion filter IFC. Moreover, even when the ionized amine runs offfrom the UF module, it can be removed by the ion filters IFA and IFC. Insuch a ultrapure water preparing system of this Embodiment, the anionremoving filter AED3 and mixed ion removing filter MED can be omitted,which contributes to simplification of this system. This makes itpossible to facilitate the maintenance of the ultrapure water preparingsystem of this Embodiment.

As illustrated in FIG. 12, the anion removing filter AED3 and mixed ionremoving filter MED in the ultrapure water preparing system of thisEmbodiment, as illustrated in FIGS. 4 and 5, may be replaced with theabove-described ion filter (second filter) IFA and ion filter (secondfilter) IFC. In FIG. 12, the heat exchanger HEXC is not illustrated.Since the ion exchange resin constituting the anion removing filter AED3and mixed ion removing filter MED contains an amine, there is apossibility of ionized amine leaking from the anion removing filter AED3and mixed ion removing filter MED when primary pure water is caused topass through these filters. The test made by the present inventors hasrevealed that ionized amine is leaked from the anion removing filterAED3 and mixed ion removing filter MED, and the leakage amount from theanion removing filter AED3 is greater. It is therefore possible to befree from an inconvenience, such as caused by a leakage of ionizedamine, when the anion removing filter AED3 and mixed ion removing filterMED are replaced with the ion filters IFA and IFC.

As illustrated in FIG. 13, the ion filter IFC may be installed in thepipe line (pathway) PL for delivering ultrapure water to the point ofuse USED from the UF equipment UFE. The point of use USEP embraces notonly cleaning and drafting equipment (first wet treating equipment) tobe used for the cleaning (first wet treatment) of the semiconductorsubstrate 1, but also chemical-solution preparing equipment (first wettreating equipment) in which a chemical solution, such as dilutehydrofluoric acid, is prepared using ultrapure water. Ionized amine is acation so that installment of the ion filter IFC in the pipe line PLmakes it possible to remove, by use of the filter IFC, ionized aminefrom ultrapure water to be fed to the point of use USEP, even if ionizedamine flows out from the UF equipment UFE as a mixture in ultrapurewater. In this Embodiment, the ion filter IFC is installed in the pipeline PL. Instead of the ion filter IFC, a mixed ion removing filter maybe used for the removal of ionized amine from ultrapure water.

The UF equipment UFE is made of a plurality of UF modules UFM, asillustrated in FIG. 14. Similar to the UF modules as described withreference to FIG. 6, these UF modules UFM have, disposed in the bodythereof, a plurality of capillary hollow fiber membranes made of apolysulfone membrane or polyimide membrane. When the plurality of hollowfiber membranes are bundled using an amine-containing adhesive, ionizedamine presumably comes to be mixed in ultrapure water discharged fromthe UF modules UFM. Therefore, in this Embodiment, the above-describedion filter IFC is disposed upstream of each of the UF modules UFM. Thismakes it possible to remove ionized amine by use of the ion filter IFCeven if ionized amine is mixed in the ultrapure water discharged fromthe UF modules UFM. Upon disposal, the capacity for allowing the passageof water is set to be greater in the ion filter IFC than in the UFmodule UFM. When the ion filter IFC is inferior with regard to thiscapacity relative to the UF module UFM, a plurality of the ion filtersIFC are disposed per UF module UFM, so that the total capacity of theplurality of ion filters IFC would exceed that of one UF module UFM.

When a plurality of hollow fiber membranes are bundled by anamine-containing adhesive, as in the above-described UF modules UFM, theamount of ionized amine is small relative to the whole amine amount. Ofthe whole amine amount, only ionized amine is hydrophilized and flowsout from the UF modules UFM, passing through the hollow fiber membranes.Most of the amine existing in the ionized form is discharged from the UFmodules together with ultrapure water after an elapse of a predeterminedterm after disposal of new UF modules in the UF equipment UFE, whichterm varies depending on the amount of water caused to pass through theUF modules. An area for installing new UF modules UFMN is established inthe UF equipment UFE, as illustrated in FIG. 15. To new UF modules UFMNinstalled in this area, as well as to the other UF modules UFM, primarypure water, which has passed through the anion removing filter AED3 andmixed ion removing filter MED, is fed. The primary pure water that isfed to the new UF modules UFMN is discharged from the new UF modulesUFMN as ultrapure water containing ionized amine existing in the new UFmodules UFMN, it is fed to the upstream side of the anion removingfilter AED3 and mixed ion removing filter MED, and then it unites withthe primary pure water upstream thereof. The ultrapure water, togetherwith the primary pure water, is then fed to the anion removing filterAED3 and mixed ion removing filter MED. The anion removing filter AED3and mixed ion removing filter MED used here each has, in the body KOT1thereof, a plurality of ion exchange resins UJ disposed as illustratedin FIG. 16. As illustrated in FIG. 17, the ion exchange resins IEF eachhas an ion exchange radical IER1, which adsorbs thereto ions in theprimary pure water fed to the body KOT1. Ionized amine contained in theultrapure water is a cation so that it can be adsorbed to and removed bythe mixed ion removing filter MED. The ultrapure water from whichionized amine has been removed is fed again to the UF equipment UFEtogether with the primary pure water. The above-described steps are thenrepeated. By these steps, new UF modules UFMN can be cleaned to removethe ionized amine existing therein, whereby primary pure water which isused for the removal of ionized amine can be provided for recycling usewithout being discarded. The test made by the present inventors hasrevealed that, when a column having a diameter of about 106 mm and aheight of 1150 mm was used as a new UF module UFMN, run-off of ionizedamine from the new UF module UFMN stopped about two or three months(preferably about 3 months) after about 3 m³ per hour of the primarypure water was caused to pass through the new UF module UFMN. Such a newUF module UFMN, which becomes free from leakage of ionized amine aftersuch a step, can be used as a substitute for an old UF module UFM. Thenumber of such new UF modules UFMN to be disposed of must be equal orgreater than that of the UF modules which are worn and need replacement.The number of new modules can be set freely depending on whether the oldUF modules UFM are replaced wholly or partially.

Alternatively, new UF modules UFMN, as described above, may be installedin a pathway (refer to FIGS. 4 and 5) for returning, to an intermediatestorage tank MIDT, a nonused portion of ultrapure water at the point ofuse USEP. Ultrapure water fed to the new UF module UFMN is dischargedtherefrom as ultrapure water containing the ionized amine existing inthe new UF module UFMN. This ionized-amine-containing ultrapure water issent to the intermediate storage tank MIDT (refer to FIGS. 4 and 5), andthere it unites with primary pure water. The ionized amine can beremoved when the primary pure water passes through the mixed ionremoving filter MED (refer to FIGS. 4 and 5). The ultrapure water fromwhich ionized amine has been removed is fed again to the UF equipmentUFE together with the primary water. The above-described steps are thenrepeated. By such steps, ionized amine existing in the new UF moduleUFMN can be removed. Moreover, ultrapure water used for the removal ofionized amine can be provided for recycling without being discarded.

As described above, the ion exchange resin IEJ (refer to FIGS. 16 and17) constituting the anion removing filter AED3 and mixed ion removingfilter MED contains an amine. The amine in the ion exchange resin doesnot contain so much ionized amine. As in the UF module UFM, the ionizedamine is hydrophilized and inevitably flows out from the anion removingfilter AED3 and mixed ion removing filter MED. It is therefore possibleto remove ionized amine contained in the ion exchange resin IEJ bydisposing new anion removing filters AED3 and mixed ion removing filtersMED in an area similar to the area in which the new UF modules UFMN aredisposed. The ionized amine contained in the ion exchange resin IEJ maybe removed, as in the case of the new UF modules UFMN, by disposing newanion removing filters AED3 and mixed ion removing filters MED in apathway (refer to FIGS. 4 and 5) for returning a nonused portion of theultrapure water at the point of use USEP to an intermediate storage tankMIDT.

When the ultrapure water prepared through the above-described steps isused in a cleaning step of the semiconductor substrate 1, from which asilicon nitride film used for the formation of the above-described fieldinsulating film 6 (refer to FIG. 3) has been removed, a cleaning anddrafting apparatus, as illustrated in FIG. 18, can be used. Theultrapure water prepared by the ultrapure water preparing system of thisEmbodiment, which has been described with reference to FIGS. 4 to 17, isfed to each of a treatment tank SC1 and pure water tanks QDR1, QDR2, OF1and OF2, which are points of use USED of ultrapure water (refer to FIGS.4 and 5). As described above with reference to FIG. 13, an ion filterIFC having an ion exchange radical capable of adsorbing thereto cationsor a mixed ion removing filter MED may be installed in respective pipelines for feeding ultrapure water to the treatment tank SC1 and purewater tanks QDR1, QDR2, OF1 and OF2. The treatment tank SC1 is fed withH₂O₂ and NH₄OH, while the treatment tank HF is fed with dilutehydrofluoric acid prepared using the ultrapure water of this Embodiment.The semiconductor substrate 1 is cleaned, as described below, by such acleaning and drafting apparatus. First, cleaning with NH₄OH/H₂O₂/H₂O isconducted in the treatment tank SC1, followed by cleaning with ultrapurewater in the pure water tanks QDR1 and OF1. After cleaning with dilutehydrofluoric acid in the treatment tank HF, cleaning with pure water isconducted in the pure water tanks QDR2 and OF2. The semiconductorsubstrate 1 is then dried by IPA (isopropyl alcohol) vapor dryingmethod, by which the step of cleaning the semiconductor substrate 1using the cleaning and drafting apparatus illustrated in FIG. 18 iscompleted. When the cleaning and drafting apparatus as illustrated inFIG. 18 is applied to another cleaning step, which does not need thetreatment in the treatment tank SC1 and pure water tanks QDR1 and OF1,the cleaning step may be started from the treatment in the treatmenttank HF.

FIG. 19 is a schematic view of a dilute hydrofluoric acid preparingapparatus. This dilute hydrofluoric acid preparing apparatus is one ofthe points of use USEP of ultrapure water (refer to FIGS. 4 and 5). Theultrapure water prepared by the ultrapure water preparing system of thisEmbodiment, as described with reference to FIGS. 4 to 17, is first fedin a predetermined amount to a pure water weighing tank TANK 1. Asdescribed above with reference to FIG. 13, an ion filter IFC having anion exchange radical capable of adsorbing thereto cations or a mixed ionremoving filter MED is installed in the pipe line for feeding ultrapurewater to the pure water weighing tank TANK 1. Undiluted hydrofluoricacid fed from a hydrofluoric acid canister CAN1 to an undilutedhydrofluoric acid tank TANK 2 is weighed by being transferred from theundiluted hydrofluoric acid tank TANK2 to a hydrofluoric acid weighingtank TANK 3. From the pure water weighting tank TANK 1 and thehydrofluoric acid weighing tank TANK 3, ultrapure water and undilutedhydrofluoric acid are then fed respectively to a blending tank TANK4, inwhich ultrapure water and the undiluted hydrofluoric acid are blended ata predetermined ratio to prepare dilute hydrofluoric acid. In thisEmbodiment, an example consists of an about 1:99 or 1:19 mixture ofundiluted hydrofluoric acid and ultrapure water. The dilute hydrofluoricacid is then transferred from the blending tank TANK4 to a feeding tankTANK5, whereby it can be provided for a cleaning and drafting apparatus.

After removal of the silicon nitride film used for the formation of thefield insulating film 6 (refer to FIG. 3) and cleaning of thesemiconductor substrate 1, the surface thereof is oxidized to formthereover a gate insulating film 8 having a film thickness of about 20nm. By wet etching with a photoresist film (not illustrated) patternedby photolithography, the gate insulating film 9 in a region 1C isselectively removed. In this Embodiment, wet etching can be carried outby a wet etching apparatus (refer to FIG. 20) which uses ultrapure waterprepared by the ultrapure water preparing system of this Embodiment, asdescribed above with reference to FIGS. 4 to 17. The ultrapure water isfed to each of the pure water tanks QDR3, OF3 and OF4, which are pointsof use USEP (refer to FIGS. 4 and 5) of the ultrapure water. Asdescribed above with reference to FIG. 13, an ion filter IFC having anion exchange radical capable of adsorbing thereto cations or a mixed ionremoving filter MED may be installed in each of the pipe lines forfeeding the pure water tanks QDR3, OF3 and OF4 with ultrapure water. Anetching tank ETCH contains an etching solution for etching of thesilicon oxide film. In such a wet etching step of the gate insulatingfilm 8 by the wet etching apparatus, the gate insulating film 8 is wetetched by immersing the semiconductor substrate 1 in the etching tankETCH. After cleaning the semiconductor substrate 1 using ultrapure waterin the pure water tanks OF3 and OF4, the semiconductor substrate 1 isdried by spin drying, whereby the wet etching step of the gateinsulating film 8 performed by the wet etching apparatus illustrated inFIG. 20 is completed.

After removal of the photoresist film, the semiconductor substrate 1 iscleaned, for example, by a cleaning and drafting apparatus, asillustrated in FIG. 21. The ultrapure water prepared by the ultrapurewater preparing system of this Embodiment, as described with referenceto FIGS. 4 to 17, is fed to each of the treatment tanks SC2 and SC3, andthe pure water tanks QDR3, QDR4, OF5 and OF6 which are each a point ofuse USEP of the ultrapure water (refer to FIGS. 4 and 5). As describedabove with reference to FIG. 13, an ion filter IFC having an ionexchange radical capable of adsorbing thereto cations or a mixed ionremoving filter MED may be installed in respective pipe lines forfeeding the treatment tanks SC2 and SC3, and pure water tanks QDR3,QDR4, OF5 and OF6. The treatment tank SC2 is fed with H₂O₂ and NH₄OH,while the treatment tank SC3 is fed with H₂O₂ and HC1 (hydrochloricacid). Cleaning of the semiconductor substrate 1 by such a cleaning anddrafting apparatus is carried out in the following manner. First,cleaning with NH₄OH/H₂O₂/H₂O is conducted in the treatment tank SC2,followed by cleaning with the ultrapure water in the pure water tanksQDR3 and OF5. After cleaning with HCl/H₂O₂/H₂O in the treatment tankSC3, cleaning with ultrapure water is conducted in the pure water tanksQDR4 and OF6. The semiconductor substrate 1 is dried by use of an IPAvapor drying method, whereby the cleaning of the semiconductor substrate1 by the cleaning and drafting apparatus as illustrated in FIG. 21 iscompleted.

As illustrated in FIG. 22, the surface of the semiconductor substrate 1is oxidized to form a gate insulating film (tunnel oxide film) 9 havinga film thickness of about 10 nm over the surface of the p type well 4 ina region 1C. The gate insulating film 9 may have a film thickness notgreater than 10 nm, for example, about 5 nm.

By CVD, a polycrystalline Si film (first conductive film) 10 of about200 nm thick is deposited over the main surface (device surface) of thesemiconductor substrate 1. This polycrystalline Si film 10 may be formedby depositing amorphous Si over the semiconductor substrate 1 by CVD andthen heat treating this amorphous Si to convert amorphous Si topolycrystalline Si.

After deposition of a phosphate glass film (not illustrated) over thesurface of the polycrystalline Si film 10, for example, by the coatingmethod, the semiconductor substrate 1 is heat treated to introduce Pinto the polycrystalline Si film 10. After removal of the phosphateglass film, the polycrystalline Si film 10 is patterned byphotolithography using a photoresist film (not illustrated) as a mask.This makes it possible to leave the polycrystalline Si film 10 in theregion 1C, and to form gate electrodes 10D, 10E2 and 10F in the regions1D, 1E2 and 1F, respectively. After removal of the photoresist film thatwas used for patterning of the polycrystalline Si film 10, heattreatment at about 950° C. is conducted to form a silicon oxide film(first insulating film) 11 over the surface of the polycrystalline Sifilm 10 (including gate electrodes 10D, 10E2 and 10F).

In the cleaning of the semiconductor substrate 1 with ultrapure water,the ultrapure water, if it contains ionized amine, inevitably etches theSi constituting the semiconductor substrate 1. As illustrated in FIG.23, which is an enlarged view of the region 1C, the interface betweenthe gate insulating film 9 and the semiconductor substrate 1 (p typewell 4) becomes uneven. This unevenness adversely affects the shape of athin film formed over the gate insulating film 9 and the interfacebetween the gate insulating film 9, and polycrystalline Si film 10 orthe interface between the polycrystalline Si film 10 and silicon oxidefilm 11 sometimes becomes uneven.

By using the ultrapure water preparing system of this Embodiment, asdescribed with reference to FIGS. 4 to 17, inclusion of ionized amine inthe ultrapure water can be prevented. Therefore, use of the ultrapurewater prepared according to this Embodiment can prevent the formation ofunevenness on the interface between the gate insulating film 9 andsemiconductor substrate 1 (p type well 4) (refer to FIG. 24). This makesit possible to prevent a lowering of the breakdown voltage of the gateinsulating film so that when a MISFET, which will constitute a memorycell of a flash memory is formed in the region 1C, deterioration in thewrite characteristics to the memory cell and the erase characteristicscan be prevented.

The present inventors measured the breakdown voltage of the gateinsulating film 9 in a manner as shown in FIG. 25. More specifically,the applied voltage, when an electric current of about 1×10⁻¹¹ is sentbetween the semiconductor substrate 1 and polycrystalline Si film 10,was measured by a probe. In FIG. 25, members other than thesemiconductor substrate 1, field insulating film 6, gate insulating film9 and polycrystalline Si film 10 are omitted. FIGS. 26 to 30 show theresults of measurement of the breakdown voltage of the gate insulatingfilm 9 at plural sites on the whole surface of the semiconductor wafer(semiconductor substrate 1). The gate insulating films 9 at sitesshowing a voltage less than 8V are regarded as defective because of adecline in breakdown voltage.

FIG. 26 shows the results of measurement of the breakdown voltage of thegate insulating film 9 that is formed after cleaning the semiconductorsubstrate 1 with the ultrapure water which has been prepared just afterreplacement of the UF modules UFM (refer to FIG. 14) of the UF equipmentUFE with new ones. In FIG. 26, results are shown for the case where theUF equipment is not equipped with the ion filter IFC illustrated in FIG.14. As described above with reference to FIG. 14, the UF modules UFMeach has, in the body thereof, a plurality of hollow fiber membranesbundled with an amine-containing adhesive. In a novel UF module UFM, aportion of an amine exists as ions. The ionized amine is hydrophilizedand flows out from the UF module as a mixture with the ultrapure water.In the cleaning of the semiconductor substrate 1 with the ultrapurewater, this ionized amine inevitably etches the Si constituting thesemiconductor substrate 1, thereby forming an unevenness on theinterface between the surface and the gate insulating film 9 formedthereover. It has been confirmed from the test results shown in FIG. 26that this unevenness lowers the breakdown voltage of the gate insulatingfilm 9.

FIG. 27 shows the results of measurement of the breakdown voltage of thegate insulating film 9 that is formed after cleaning the semiconductorsubstrate 1 with the ultrapure water which has prepared just after thereplacement of the anion removing filter AED3 and mixed ion removingfilter MED (refer to FIGS. 4 and 5) with new ones. Similar to theresults shown in FIG. 26, FIG. 27 shows the results obtained in the casewhere the ion filter IFC illustrated in FIG. 14 is not installed. Asdescribed above with reference to FIG. 12, the ion exchange resinconstituting the anion removing filter AED3 and mixed ion removingfilter MED contains amines, so that when the primary water passesthrough these filters, ionized amine runs out from them. This ionizedamine is hydrophilized and discharged from the UF equipment UFE as amixture with the ultrapure water. Similar to the results shown in FIG.26, this ionized amine inevitably etches the Si constituting thesemiconductor substrate 1, thereby forming an unevenness on theinterface between its surface and the gate insulating film 9 formedthereover. It has been confirmed from the test results shown in FIG. 27that this unevenness lowers the breakdown voltage of the gate insulatingfilm 9.

FIG. 28 shows the results of measurements of the breakdown voltage ofthe gate insulating film 9 that is formed after the cleaning of thesemiconductor substrate 1 with ultrapure water prepared using the UFmodules UFM of the UFE equipment UFE which have been used for a longtime (for example, at least about 3 months). Similar to the resultsshown in FIG. 26 or FIG. 27, FIG. 28 shows results in the case where theion filter IFC illustrated in FIG. 14 is not installed. As describedabove with reference to FIG. 15, most of the amines existing in ionizedform are discharged, together with the ultrapure water, from the UFmodules after an elapse of a predetermined term, which varies dependingon the amount of water caused to pass through the UF modules UFM. Thereis no possibility of the ionized amine flowing out from the UF modulesUFM if they have been used for a long time so that unevenness on theinterface between the semiconductor substrate 1 and the gate insulatingfilm 9, which will otherwise be formed as a result of etching, by theionized amine, of the Si constituting the semiconductor substrate 1,does not occur. It can be confirmed also from the test results shown inFIG. 28 that the gate insulating film 9 is free from a reduction in thebreakdown voltage because such an inconvenience is inhibited.

FIG. 29 shows the results of measurements of the breakdown voltage ofthe gate insulating film 9 that is formed after cleaning thesemiconductor substrate 1 (refer to FIG. 13) with the ultrapure waterprepared by replacing the UF modules UFM with new ones and disposing amixed ion removing filter MED downstream of the UF equipment UFE. Inthis case, the ionized amine flowing out from the new UF modules UFM canbe removed by the mixed ion removing filter MED, so that unevenness onthe interface between the semiconductor substrate 1 and the gateinsulating film 9, which will otherwise be formed as a result ofetching, by the ionized amine, of the Si constituting the semiconductorsubstrate 1, does not occur. It can be confirmed also from the testresults shown in FIG. 29 that the gate insulating film 9 is free from areduction in the breakdown voltage because such an inconvenience isinhibited.

FIG. 30 shows the results of measurement of the breakdown voltage of thegate insulating film 9 that is formed after cleaning the semiconductorsubstrate 1 (refer to FIG. 13) with the ultrapure water prepared byreplacing the UF modules UFM with new ones and disposing an ion filterIFC (refer to FIG. 11) downstream of the UF equipment UFE. In this case,the ionized amine flowing out from the new UF modules UFM can be removedby the ion filter IFC so that unevenness on the interface between thesemiconductor substrate 1 and the gate insulating film 9, which willotherwise be formed as a result of etching, by the ionized amine, of theSi constituting the semiconductor substrate 1, does not occur. It can beconfirmed also from the test results shown in FIG. 30 that the gateinsulating film 9 is free from a reduction in the breakdown voltagebecause such an inconvenience is inhibited.

FIG. 31 illustrates the relationship, classified by specification,between the amount of ionized amine attached to the semiconductorsubstrate 1 as a result of cleaning with ultrapure water and theexistence or absence of a defective gate insulating film 9, asdetermined by the testing method illustrated in FIG. 25. At this time,the ultrapure water is fed to a cleaning and drafting apparatus (referto FIG. 18) at a rate of 15 liter/minute. The cleaning step is startedwith the treatment in the treatment tank HF. After removal of the gateinsulating film 8 that is formed in the region 1C to expose the Siconstituting the semiconductor substrate 1, treatment is conducted inthe pure water tanks QDR2 and OF2. Since the amount of ionized aminemixed in the ultrapure water was only a trace, the treatment time in thepure water tank OF2 was adjusted to about 100 minutes so that the amountof ionized amine attached to the semiconductor substrate 1 is clearlydifferent among the specifications of the ultrapure water preparingsystem. The item indicated as “P test” shows the test results by thetesting method (which will hereinafter be called a “probe test”)illustrated in FIG. 25. Among the specifications of the ultrapure waterpreparing system to be tested, the specification indicated by “ReF” hasUF modules UFM of the UF equipment UFE, which modules have been used fora long period of time (for example, about three months or greater). Thespecification indicated by “new UF” has new UF modules as the modules ofthe UF equipment UFE. The specification 1 indicates UF equipment UFEinstalled with new UF modules UFM after cleaning for about 2 weeks in asimilar manner to that described above with reference to FIG. 15. Thespecification 2 indicates UF equipment UFE installed with new UF modulesafter cleaning for about 6 weeks in a similar manner to that describedabove with reference to FIG. 15. The specification 3 indicates UFequipment UFE which is installed with new UF modules and has a cationdeminer and an anion deminer disposed downstream of the UF equipment UFE(between the UF equipment UFE and point of use USEP). The specification4 indicates UF equipment UFE (FIG. 11), which is installed with new UFmodules UFM and has an ion filter IFA and an ion filter IFC disposeddownstream of the UF equipment UFE (between the UF equipment UFE andpoint of use USEP). As a result of comparison of the attached amount ofionized amine among these specifications, supposing that the attachedamount of ionized amine in ReF is 100, the amount is greater in the newUF, Specification 1, Specification 2 and Specification 3 than in ReF.According to the results of the probe test, the new UF and specification1 are judged to be defective. Although the specification 2 is judged tobe defective as a result of the probe test, its defectiveness is lighterthan that of the new UF or specification 1. From these results, theeffectiveness of the ultrapure water preparing system of thisEmbodiment, characterized in that ionized amine is removed by the UFequipment and an ion exchange resin type ion removing filter (ion filterIFA and ion filter IFC) or deminer (cation removing filter and anionremoving filter) disposed downstream of the UF equipment UFE (betweenthe UF equipment and the point of use USEP) has been confirmed, alongwith the effectiveness of the use of new UF modules UFM, as illustratedin FIG. 15, after cleaning for a predetermined term.

FIG. 32 illustrates the relationship between the percent defectives ofthe gate insulating film 9 and the date on which the semiconductorsubstrate 1 was cleaned with ultrapure water reckoned from the date onwhich the UF modules UFM of the UF equipment UFE are replaced with newones. At the time when mass production (cleaning step) of the flashmemory of this Embodiment is started again after replacement of the UFmodules UFM with new ones, a TOC content in the ultrapure water, itsspecific resistance and the concentration of dissolved oxygen in theultrapure water have recovered their normal values, more specifically,approximately 1.0±0.2 ppb, 18.25 MΩ and 20±3.0 ppb, respectively.Although the mass production is restarted when this TOC content,specific resistance and concentration of dissolved oxygen become normalvalues, some gate insulating films 9 are defective, suggesting thatthese elements have no relationship with the percent of defective gateinsulating films 9. The TOC content, specific resistance and dissolvedoxygen concentration, each of the ultrapure water, become theabove-described values for about 1.5 days, about 0.5 day and about 0.5day after replacement of the UF modules UFM with the new ones, so thatfor about three days after replacement with the UF modules UFM with newones, the cleaning step is not conducted even though ultrapure water isprovided. The percent of defective gate insulating films 9 increasesjust after the day of replacement of the UF modules UFM with new ones,followed by a gradual decrease day by day. This occurs because, as theprimary pure water passes through the new UF modules UFM, the ionizedamine existing in the new UF modules UFM flows out and its amountdecreases. From the above-described results, the effectiveness of thecleaning of new UF modules UFM for a predetermined term, as illustratedin FIG. 15, can be confirmed.

As illustrated in FIG. 33, a silicon nitride film 13, a silicon oxidefilm 14 and a silicon nitride film 15 are successively stacked over thesemiconductor substrate 1. The silicon nitride films 13,15 are formed,for example, by deposition by CVD, while the silicon oxide film 14 isformed, for example, by heat treating the semiconductor substrate 1. Thesilicon oxide films 11,14 and silicon nitride films 13,15 are called aninterlayer capacitor film 16, collectively. Using a photoresist film(not illustrated) patterned by photolithography as a mask, theinterlayer capacitor film 16 is dry etched to remove the interlayercapacitor film 16 from the regions 1A, 1B.

Over the surface of the p type well 4 in the region 1A and the surfaceof the n type well 3 in the region 1B, a silicon oxide film (notillustrated) is formed by oxidizing treatment. Then, into the p typewell 4 in the region 1A and n type well 3 in the region 1B, BF₂ isintroduced.

After removal of the photoresist film that is used for dry etching ofthe interlayer capacitor film 16, the surface of the semiconductorsubstrate 1 is oxidized to form, for example, a gate insulating film 17of about 13.5 nm thick over the surface of the p type well 4 in theregion 1A and the surface of the n type well 3 in the region 1B, asillustrated in FIG. 34.

Over the main surface of the semiconductor substrate 1, apolycrystalline Si film (second conductive film) 18, WSi_(x) film(second conductive film) 19 and silicon oxide film 20 are stackedsuccessively. After the deposition of the polycrystalline Si film 18, aphosphate glass film (not illustrated) may be deposited by the coatingmethod, followed by heat treatment of the semiconductor substrate 1 tointroduce P into the polycrystalline Si film 18.

As illustrated in FIG. 35, using a photoresist film (not illustrated)that has been patterned by photolithography as a mask, the silicon oxidefilm 20 is patterned. After removal of the photoresist film, the WSi_(x)film 19 and the polycrystalline Si film 18 are dry etched using thesilicon oxide film 20 as a mask. By this step, in the regions 1A and 1B,gate electrodes 29A,29B made of the WSi_(x) film 19 and thepolycrystalline Si film 18 can be formed, respectively, while in theregion 1C, a control gate electrode 22, that is made of the WSi_(x) film19 and the polycrystalline Si film 18, can be formed. In the regions1E2,1D,1F, the interlayer capacitor film 16 is etched, while leaving thesilicon nitride film 13.

As illustrated in FIG. 36, the polycrystalline Si film 10 is dry etchedusing the silicon oxide film 20 as a mask in the region 1C, whereby afloating gate electrode 24 can be formed. A region other than the region1C is covered with the photoresist film so that exposure to the etchingatmosphere can be prevented. Here, the floating gate electrode 24,interlayer capacitor film 16 and control gate electrode 22 are calledthe gate electrode 25, collectively. Oxidizing treatment is thenconducted to form a thin silicon oxide film 30 on the side walls andupper surfaces of the gate electrodes 25, 29A and 29B.

As illustrated in FIG. 37, using a photoresist film (not illustrated)that has been patterned by photolithography as a mask, an n typeimpurity (for example, P) is introduced into the p type well 4 on oneside of the gate electrode 25 by ion implantation, followed by heattreatment.

After removal of the photoresist film, a new photoresist film (notillustrated) is formed over the regions 1A, 1C, 1E2 and 1D. Using thisphotoresist film as a mask, a p type impurity (for example, BF₂) isintroduced into the n type well 3 by ion implantation, whereby a p⁻ typesemiconductor region 31 is formed.

After removal of the photoresist film from the regions 1A, 1C, 1E2 and1D, another photoresist film (not illustrated) is formed over theregions 1B and 1F. Using the photoresist film as a mask, an n typeimpurity (for example, P) is introduced into the p type well 4 by ionimplantation to form an n⁻ type semiconductor region 32. Then, thephotoresist film is removed from the regions 1B and 1F.

As illustrated in FIG. 38, a silicon oxide film is then deposited overthe semiconductor substrate 1 by CVD. By anisotropic etching of thesilicon oxide film, side wall spacers 33 are formed by leaving thesilicon oxide film on the side walls of the gate electrodes 29A, 29B,25, 10E2, 10D and 10F.

A photoresist film (not illustrated) is then formed over the regions 1Band 1F and over the gate electrodes 29A, 25, 10E2 and 10D, so as tocover, with the photoresist film, a predetermined range of the n⁻ typesemiconductor region 32 on one side of the gate electrode 10D. Using thephotoresist film as a mask, an n type impurity (for example, P) is thenintroduced into the p type well 4 by ion implantation.

After removal of the photoresist film, another photoresist film (notillustrated) is formed over the regions 1A, 1C, 1E2 and 1D and over thegate electrodes 29B and 10F, so as to cover, with the photoresist film,a predetermined range of the p⁻ type semiconductor region 31 on one sideof the gate electrode 10D. Using the photoresist film as a mask, a ptype impurity (for example, BF₂) is then introduced into the n type well3 by ion implantation. After removal of the photoresist film, thesemiconductor substrate 1 is heat treated at about 900° C., whereby a p⁺type semiconductor region 34 and n⁺ type semiconductor regions 35 and35A are formed. By these steps, a 5V type nMISQA, a 5V type pMISQB, aMISQC which will constitute a memory cell of a flash memory, ahigh-breakdown-voltage loading nMISQE2, a high-breakdown-voltageone-side offset nMISQD and a high-breakdown-voltage one-side offsetpMISQF can be formed in the regions 1A, 1B, 1C, 1E2, 1D and 1F,respectively.

As illustrated in FIG. 39, a silicon oxide film 36 of about 150 nm thickis then deposited over the semiconductor substrate 1 by CVD. The siliconoxide film 36 is then dry etched using a photoresist film (notillustrated) that has been patterned by photolithography as a mask,whereby a contact hole 38A reaching the n⁺ type semiconductor region 35Ais formed in the silicon oxide film 36.

After removal of the photoresist film, an amorphous Si film is depositedover the semiconductor substrate 1 by CVD, thereby embedding the contacthole 38A with the amorphous Si film. A polycrystalline Si film is thenformed by heat treatment of this amorphous Si film. By dry etching usinga photoresist film (not illustrated) that has been patterned byphotolithography as a mask, the polycrystalline Si film is patterned toform an interconnect TG. The semiconductor substrate 1 is then heattreated, whereby a silicon oxide film 36A is formed over the surface ofthe interconnect TG.

As illustrated in FIG. 40, a BPSG film 37 is deposited over thesemiconductor substrate 1 by CVD, followed by heat treatment of thesemiconductor substrate at about 900° C. in an N₂ atmosphere toplanarize the surface of the BPSG film 37.

Through a photoresist film (not illustrated) that has been patterned byphotolithography, the BPSG film 37, silicon oxide film 36 and gateinsulating films 8,17 are dry etched, whereby a contact hole 38 isformed.

After removal of the photoresist film used for perforation of thecontact hole 38, an MoSi (molybdenum silicide) film of about 30 nm thickis deposited in the contact hole 38 and over the BPSG film by sputteringto form a barrier conductor film. Over the barrier conductor film, ametal film to embed therewith the contact hole 38 is deposited bysputtering. This metal film is composed mainly of Al (aluminum) andcontains Cu (copper). An antireflective film is then formed bydepositing an MoSi film over the metal film. The barrier conductor filmhas a function of preventing diffusion of the Al in the metal film intothe BPSG film 37 and silicon oxide film 36, while the antireflectivefilm serves to prevent irregular reflection upon formation of aphotoresist film over the antireflective film in the subsequent step.

By dry etching through a photoresist film (not illustrated) that hasbeen patterned by photolithography, the antireflective film, metal filmand barrier conductor film are patterned to form an interconnect 39,whereby the flash memory of this Embodiment is fabricated.

The present invention so far has been described specifically based onembodiments of the invention. It is needless to say that the presentinvention is not limited to the embodiments, but can be modified to anextent not departing from the gist of the invention.

For example, use of ultrapure water prepared in the above-describedembodiment for a cleaning step of a semiconductor substrate duringfabrication of a flash memory was described, but it can be applied to acleaning step employed during the fabrication of a semiconductorintegrated circuit device (for example, logic circuit), other than aflash memory.

Of the features of the inventions disclosed by the present application,effects available by representative ones will be described simply below.

It is possible to prevent run-off of ionized amine into the ultrapurewater in the step of preparing ultra pure water to be used for thefabrication of a semiconductor integrated circuit device, making itpossible to prevent a lowering of the breakdown voltage of a gateinsulating film, which will otherwise occur due to the formation ofunevenness at the interface between the gate insulating film and thesemiconductor substrate.

1. A method of fabrication of a semiconductor integrated circuit device,comprising the steps of: (a) introducing first water into a first waterpurifying system, and sending out first purified water from said firstwater purifying system; (b) introducing said first purified water into asecond water purifying system having a pure water circulating loopincluding an ultrafiltration filter having a hollow fiber typeultrafiltration membrane; and sending out second purified water from afirst supply point on said pure water circulating loop; (c) providing afirst wet treatment apparatus with said second purified water, therebycarrying out a first wet treatment to a wafer in said first wettreatment apparatus, wherein step (c) including the sub-steps of: (c1)removing ions from said second purified water through use of an ionremoving filter, at least capable of removing cations, disposed betweensaid first supply point and a use point in said first wet treatmentapparatus, and (c2) providing said use point with said second purifiedwater which has passed through said ion removing filter, and wherein (i)said ion removing filter is a membrane type ion filter having a membranesheet as a filter member; (ii) said semiconductor integrated circuitdevice has a flash memory cell; and (iii) said first wet treatment is acleaning treatment prior to a thermal treatment for forming a gateinsulating film of said flash memory cell.
 2. A fabrication methodaccording to claim 1, wherein said semiconductor integrated circuitdevice includes an MISFET having said gate insulating film whosethickness is dielectrically equivalent to a silicon oxide film of 20 nmor less.
 3. A fabrication method according to claim 1, wherein saidsemiconductor integrated circuit device includes an MISFET having saidgate insulating film whose thickness is dielectrically equivalent to asilicon oxide film of 10 nm or less.
 4. A fabrication method accordingto claim 1, wherein said semiconductor integrated circuit deviceincludes an MISFET having said gate insulating film whose thickness isdielectrically equivalent to a silicon oxide film of 5 nm or less.
 5. Afabrication method according to claim 1, wherein said first wettreatment is a rinsing treatment prior to said thermal treatment to formsaid gate insulating film of said flash memory cell.
 6. A fabricationmethod according to claim 5, wherein a flow channel has no branchesbetween said ion removing filter and said use point.
 7. A fabricationmethod according to claim 2, wherein said pure water circulating loopstarts from an intermediate tank accepting said first purified water,and returns to said intermediate tank.
 8. A fabrication method accordingto claim 5, wherein said pure water circulating loop starts from anintermediate tank accepting said first purified water, and returns tosaid intermediate tank.